How to calculate the median of each 7 numbers out of a list of 1800

I have a sensor that updates every 2 seconds. That means I have 30 reads/minute, and 1800 reads/hour. For viewing the reads I wanted to use a multislider which is limited to 256 bars, but anyways, 1800 bars would’ve been too much!

So I figure that 1800 reads / 256 bars = ~7 reads per bar (7.03125 to be more exact)

I wanted to calculate the median of each series of 7 reads but that would be 256 calculations and with something like [zl slice] and [zl median] I realize it would be too much!

Does anybody know of a simpler solution?
Thank you

PS: I also thought about reducing the original number of reads but that would beat the purpose as I really want to take into consideration each and every one of the reads.